The shift was not epic, and the change isn't particularly dramatic.
The site very early on attracted the early adopters that really enjoyed the game aspect of the system, and many of them stuck around. Since that time the site has gathered the other 90% of programmers who know it now as a good place to get quick answers, but aren't all that interested in the game aspect.
If you correctly and quickly answer 20-40 questions a day, you too will be able to get reputation quickly. For instance, falsetru has been around for only 10 months, but during that time answered over 2,500 questions - an average of 8 a day. He's at 66,000+ reputation - all for answering an average of 8 questions each day.
Jon Skeet answered over 15 in the last 24 hours. Anubhava answered nearly 40 in the last 24 hours.
That may seem modest, but consistency is key. Some people like to claim that it was easier to get rep in the past, and that's why old-timers have so much rep, but if you look at any old timer you find that they have thousands and tens of thousands of answers.
Jon Skeet isn't at 600k+ rep because he got most of it early on, he's at that level because he has answered an average of over 25 questions every day for the last five years, and he continues to do so.
There are slower tags to earn rep in, but even there you will find that there is a direct correlation between number of questions answered and reputation, and almost no correlation between being an early user of Stack Overflow and reputation in the absence of a high rate of answers.
You can earn just as much reputation today as a person could in the past. The things that gather reputation most efficiently have changed slightly - questions aren't as profitable as they used to be, for instance.
Providing a large quantity of consistently good answers quickly will always pay off in terms of reputation.
If you want a formula to getting to 20k as quickly as possible, it's this:
- Answer a new question with a correct, well written but complete, short answer.
- Have you hit the reputation cap yet? If no, return to 1.
- Answer 20 more questions before the end of the GMT day.
- When the GMT day is over, return to 1.
You will reach 20k within 3 months, and this was true in the early days, has been true since then, and is still true now. Ignore all the other activities on the site. Don't worry about reviews, address comments minimally but don't invest more time that you could be spending on another answer.
This takes time, assuming you spend 2-3 minutes on each answer, you're dedicating at least 2 hours a day just to answering questions and achieving your goal. It's not cheap or easy, those who have high reputation aren't simply sitting on their hands, eating magical manna, they are working hard, spending hours a day, helping others with their questions.